Where do the collections go in 2.8+ when you hit Ctrl-G to make a group?
Okay after a lot of digging it appears that Ctrl-G not creating a usable collection is actually known by the developers: Blender developer discussion thread.

Some of the devs on that thread say it needs to be fixed since the "Create Group" hotkey does nothing perceivable by the user. Others appear to want to leave it the way it is so that it can be used for a new "Common groups" feature when it is implemented. The discussion has been going on for about half a year and the current status appears to be that Ctrl-G does not do what one would expect from reading the documentation. At least I'm not missing something obvious.